Metallurgical grade silicon contains several metallic and non-metallic impurities which make it unsuited for use in solar cells. The non-metallic impurities such as boron and phosphorus can be reduced mainly by choice of suitable raw materials for production of silicon metal, but this is only possible to a certain degree as regards the most important metallic impurities Fe, Al, Mn, Cu, Ni and others. Raw materials of high purity are however very expensive and it is therefore desirable to procure a simple and cheap purification process by which the metallic impurities can be removed and/or reduced to such a low degree that the purified silicon is suitable for production of solar cells.
It is known that a number of the metallic impurities are rejected by the silicon metal during the crystallization, and that they will crystallize along the grain borders of the silicon, either as intermetallic compounds or as silicides. This is particularly the case for iron and aluminum. The purification of silicon can therefore be done by effecting the crystallization so that the impurities can be collected and removed, for instance by means of crystal drawing, zone melting or similar methods, or by dissolving the impurities by means of an acid which does not attack the silicon metal.
Crystal drawing as well as zone melting are very effective purification methods, but are extremely expensive and require at least double purification of the metallurgical grade silicon before satisfactory solar cell quality is obtained.
From U.S. Pat. No. 4,539,194 it is known a method where molten silicon is alloyed with 1-10% by weight of calcium whereafter the solidified alloy is leached in two steps. In the first leaching step it is used an aqueous solution of FeCl3 and HCl which causes disintegration of the silicon into small silicon particles. In the second leaching step it is used an aqueous solution of HF and HNO3. By this method it is obtained a good purification for both iron and aluminum and also to some extent also for phosphorus. The method according to U.S. Pat. No. 4,539,194 does, however, have some drawbacks. Thus the addition of relatively large amounts of calcium to silicon is costly as the calcium losses during alloying are high and increases with increasing amount of calcium in the silicon. Further, it is difficult to control the leaching reactions in both leaching steps due to high heat evolution and due to formation of silane and H2-gas which can cause self-ignition and explotion. Finally, the high amount of calcium in the silicon causes a relatively high loss of silicon in the form of very fine particulate silicon which are lost in the washing steps carried out after the leaching steps.